Prime Mover - Gas Turbine Efficiency

By the way, I also wonder how can prime mover power ratings can be upgraded?? I mean how can their ratings be increased if needed??

The gas turbine send in a fixed quantity (volume) of air from ist compressor stage to the combustion stage.

When the ambient temperature (and hence the inlet air temperature) increases then its density goes down. And that creates a non optimal combustion. And that also is compounded by the (though much lesser) associated efficiency loss in the compressor stage.

The degree of non-optimality depends of course on the design.

You are correct. Raising the inlet air temperature will increase the efficiency, but decrease the actual output. This is why gas turbines are often fitted with a "regenerator" which extracts heat from the exhaust gases and feeds it back to the intake. This lowers the specific fuel consumption, but also lowers the output power. If you wanted the highest output per lb. weight of engine (as in an aircraft), you would not use a regenerator. If you wanted the lowest fuel consumption (as in a ship), you would use a regenerator. I am mindful of the results achieved with the Rover-BRM gas turbine racing car at Le Mans:

"For the 1964 season, the major change was the addition of a pair of ceramic rotary regenerators to the gas turbine, so as to improve efficiency. Although often reported as "heat exchangers", these use a different technique. Two honeycomb disks rotate slowly at 20 rpm with both inlet and exhaust airflows passing through them, but separately. This heats the disk, which then rotates, and in turn heats the inlet air. Regenerators slightly restrict the peak power of a gas turbine, but hugely increase its efficiency in compensation."

The modified engine doubled the miles per gallon! So there you go.

In case of Industrial (Power generator) Gas Turbines, this concept becomes non-practical, and I have not seen usage of this. Since you have several other complications coming in with the hot inlet air along with the one mentioned above.

There were some designs with the tube exchangers (like the air pre-heater systems in the thermal power stations). But the drop in the exchanger must be very low on bot sides - suction (for turbine) as well as exhaust.

Though there were a bit improvement in efficiency, the power available on the shaft more than negated it.

The cost of the exchanger was another negative.

The control system has to ensure that the exhaust temperature is higher than the compressor outlet (ie the combustion chamber inlet) temperature.

The Air from compressor are tapped and used for the nozzle and casing cooliing purposes.

The far better methods are to use the exhaust heat to run a steam turbine in Combined Cycle Plants.

In this case where the exhaust gas is used to drive another turbine, then the cooler inlet air will be at best...

Cold air is more dense.

Check out why gas turbine Helicopters have problems at higher altitudes and hot weather.

There is a more efficient regenerator (heat exchanger) on the market that utilizes a brazed stainless steel core www.palconltd.com